Neuroendocrine peptides serve many important functions in cell-cell signalling. The discovery of bioactive peptides has typically required difficult purification procedures. We have developed a novel technique that uses a single affinity column step to isolate peptides that are substrates of carboxypeptidase E, one of the key enzymes involved in the processing of peptide precursors into the mature forms. Using this technique we sequenced over 100 peptides. Many of these are known peptides, which validates the approach. A large number of the peptides we identified represent novel cleavage products of known neuroendocrine proteins. In addition, we identified five peptides which are derived from a single novel precursor, termed proSAAS. Based on preliminary data, we hypothesize that proSAAS is an endogenous inhibitor of prohormone convertase I (a neuropeptide processing enzyme), and that smaller fragments of proSAAS have additional functions in cell-cell signalling. To test this hypothesis, we will pursue the following specific aims: Aim 1 will investigate the processing of proSAAS in rat brain and other tissues. Aim 2 will investigate the distribution of pr0SAAS and proSAAS-derived peptides in rat brain and other tissues using immunohistochemistry. . Aim 3 will study proSAAS and the proSAAS-derived peptides as enzyme substrates and inhibitors. Aim 4 will explore functions for pr0SAAS-derived peptides in cell-cell signalling. The studies described in this application will advance our knowledge of peptide processing. The evidence that pr0SAAS functions as an endogenous inhibitor of prohormone convertase 1 implies that pr0SAAS will be a factor in the processing of numerous peptides, and thus the proposed studies are likely to be relevant to human health and disease; neuroendocrine peptides have been implicated in the regulation of pain, memory, feeding, metabolism, circadian rhythms, anxiety, and many other behaviors.